Method and apparatus for combined stretch and die forming of metal stock



Nov. 22, 1960 c. J. BATH METHOD AND APPARATUS FOR COMBINED STRETCH ANDDIE FORMING OF METAL s'rocx 7 Sheets-Sheet 1 Filed March 17, 1960 lllEkINVENTOR. MI BY i m-me/wx Nov. 22, 1960 c. J. BATH METHOD AND APPARATUSFOR COMBINED STRETCH AND DIE FORMING 0F METAL STOCK 7 Sheets-Sheet 2Filed March 1'7, 1960 IN V EN TOR. 7%

f ATTORNEY.

Nov. 22, 1960 c; J. BATH 2,961,028

METHOD AND APPARATUS FOR COMBINED STRETCH AND DIE FORMING OF METAL STOCKFiled March 17, 1960 7 Sheets-Sheet 3 "\1 I T K I II I INVIENTOR. 6904:}M, BY

rArroeA/fx Nov. 22, 1960 c. J. BATH 2,961,028 METHOD AND APPARATUS FORCOMBINED STRETCH AND DIE FORMING OF METAL STOCK Filed March 1'7, 1960 7Sheets-Sheet 4 INVE'IYTOR. Fig 5 W j 2 @Arm/e/ve'x I NOV. 22, 1960 c, JBATH METHOD AND APPARATUS FOR COMBINED STRETCH AND DIE. FORMING 0F METALSTOCK Filed March 1'7, 1960 7 Sheets-Sheet 5 INVENTOR. W 2 BY 1"ATTOENEK METHOD AND APPARATUS FOR COMBINED STRETCH AND DIE FORMING OFMETAL STOCK Filed March 17, 1960 7 Sheets-Sheet 6 IN V EN TOR.

94 64 BY 2L Nov. 22, 1960 c. J. BATH 2,961,028

METHOD AND APPARATUS FOR COMBINED STRETCH AND DIE FORMING OF METAL STOCKFiled March 17, 1960 7 Sheets-Sheet 7 INVENTOR. M

A ATTOENEK United States Patent WIE'I'I-IOD AND APPARATUS FOR COMBINEDSTRETCH AND DIE FORMING 0F METAL STOCK Cyril J. Bath, Chagrin Falls,()hio Filed Mar. 17, 1960, Ser. No. 15,588

14 Claims. (Cl. 153-35) This invention relates to a method and apparatusfor forming metal stock, and particularly for forming sheet metal stockinto articles between the complementary matching surfaces of drawingdies.

One of the principal objects of the present invention is to preconditionmetal stock, immediately prior to formation thereof between matchingsurfaces of drawing dies, and independently of die operation, so as torender it more readily fiowable and plastic, whereby it can be made toconform more accurately to the matching die surfaces upon closure of thedies with far less pressure than heretofore required for die forming.

Another object is thus to precondition the stock wherefore the producedarticles have physical properties superior to the same articles whenshaped by the conventional methods of forming in matching drawing dies.

A more specific object of the invention is to precondition the stock,immediately prior to, and independently of, closure of the matchingdrawing dies and the formation of the article thereby, by stretching thestock into a range above its elastic limit and below its ultimatestrength, whereby it is rendered more flowable and plastic and, promptlyupon its thus being rendered more flowable and plastic and before workhardening, forming it between matching surfaces of the drawing dies.

Another specific object is to effect the final formation in drawingdies, after prestretching and while the stock is thus softened, byapplying the dies to the stock progressively from one end of the dies tothe other so that, during forming, the metal between the dies can flowgradually from the portion instantaneously being engaged to theunengaged portion and thus accommodate itself more readily to the diesurfaces line at a time while the already engaged portion remainsconfined, by the die surfaces, whereby wrinkles and entrapped airpockets are eliminated and better physical properties, greater accuracyin shape, less spring back, and greatly reduced power requirements ascompared to the conventional practices of drawing metals into shape inmatching drawing dies, are obtained.

Another specific object is to form large metal sheets into shallowspheroidal segments of large surface area, such as automobile toppanels, side panels, engine and rear compartment cover panels, and thelike, wherein the depth of the segment or bulge is so limited inrelation to the area that the major portion of the metal stock cannot bedrawn and formed effectively by conventional drawing die operationsbecause it remains to a large ex tent in its elastic deformation rangein the closed positions of the dies and hence does not take the finalshape of the die surfaces but returns toward its original shape.

Another specific object is to form sheet metal stock by tensioning thestock in a range above its elastic limit unidimensionally, asdistinguished from so tensioning it concurrently endwise and crosswise,prior to and independently of the operation of the drawing dies, and,while maintaining the stock thus tensioned in said range,

effecting closure of the dies progressively in the tensioning direction,as distinguished from transversely of the tensioning direction, from onestock margin which extends transversely of the tensioning direction tothe opposite stock margin, while leaving those margins of the stockwhich extend in the tensiom'ng direction free to creep laterally betweenthe matching faces intothe dies in a direction transversely of thetensioning direction, and while maintaining each increment of the stockconfined between the matching faces from the instant that it isinitially confined therebetween until the end of the die formingoperation so that the major portion of the finally shaped article isdisposed within the peripheral limits of the matching surfaces, in thefinal forming position of the dies, and in that final position isengaged over a large portion of its surface area with the entire areasof the matching faces.

Another object is to provide apparatus especially adapted to perform diesteps of the methods herein disclosed.

Various other objects and advantages of the invention will becomeapparent from the following description, wherein reference is made tothe drawings illustrating preferred embodiments of the invention, andwherein:

Fig. 1 is a perspective view illustrating a preferred embodiment of theinvention;

Fig. 2 is a front elevation, partly in section, of the machine asillustrated in Fig. 1;

Fig. 3 is a right end elevation, partly in section, of the main frame,rocker arm and dies of the machine illustrated in Pig. 2, the othermechanisms being omitted;

Fig. 4 is a top plan view of one part of the stretch forming mechanismused in the present machine;

Pig. 5 is a side elevation of the apparatus illustrated in Fig. 4, partthereof being shown in section for clearness in illustration;

Fig. 6 is a top plan view of the complementary part of the stretchforming mechanism, part thereof being shown in section for clearness inillustration;

Fig. 7 is a side end elevation of the structure shown in Fig. 6;

Fig. 8 is an enlarged vertical axial sectional view of one of the setsof gripping jaws illustrated in Figs. 6 and 7;

Fig. 9 is a diagram illustrating a hydraulic circuit for operating themachine;

Fig. 10 is a perspective view illustrating a modification of the machineillustrated in Fig. 1;

Fig. 11 is a perspective view of a modified arrangement of the dies forthe machines illustrated in Figs. 1 or 10; and

Fig. 12 is a front elevation of a modified form of dies illustrated inFig. 11.

For purposes of illustration, the method and apparatus are describedhereinafter as applied in the formation of present day automobilepanels, such as top, side, and door panels, engine compartment covers,and rear compartment covers, as the formation of panels of this typepresents special difficulties which can be overcome readily by thepresent method and apparatus.

Such a panel usually is a large sheet which has been formed into a veryshallow spheroidal segment having a large surface area and a depth solimited in relation to the area that the major portion of the stockcannot be drawn into final shape and formed effectively by conventionaldrawing die operations. This is because that major portion remains belowthe elastic limit of the stock and, therefore, only within the elasticdeformation range, throughout die closure and even in the closedposition of the dies.

In the conventional drawing of these panels, the periph- 2,961,028 a i Wcry of the sheet is engaged by the press hold-down or clamping means andthe entire area within the peripheral limits of the dies is drawn byapplying the dies, through the entire drawing operation, parallel totheir final closed position. As a result, most of the stretching anddrawing occurs only at a very narrow zone just inside the peripheryengaged by the holddown means. Quite-often, the metal in this zone isoverstretched and thinned while the more central major portion of thesheet is not permanently stretched significantly. Instead, theunstretched central portion is merely compressed and work hardened witha resultant deterioration in its physical properties.

Because of these phenomena, consistent high quality and dimensionalaccuracy are not often obtained and hand finishing often is necessary.

As mentioned, the present method generally comprises stretching a lengthof sheet metal stock, preferably unidimensionally,independently of, andprior to, the die drawing operation, into a range above its elasticlimit, then promptly engaging it between the matching faces of drawingdies, while the metal is still in a plastic flowable condition due tohaving been previously stretched into a range above the elastic limit ofthe stock, and

before work hardening has resulted from the stretching operation. 7

In the case of some metals and shapes, the tensioning force may bediscontinued before engagement of the sheet between the dies, if the dieforming follows the stretching in the few seconds during which theplasticity due to stretching continues and before work hardeningresults, but preferably the tensioning force is continued by meansindependent of the dies, during closure.

Furthermore, it is desirable that the unidimensional tensioning into therange above the elastic limit be employed, as in the Dolney and HeinPatent No. 2,810,421, issued October 22, 1957, or in the Peter P.Matthews application Ser. No. 757,973, filed August 29, 1958, asdistinguished from stretching rectangular sheets by simultaneouslytensioning them endwise and laterallyby stretching devices connected toall four margins.

The dies are preferably applied progressively, line at a time in thedirection of tensioning, from one stock margin which extendstransversely of the tensioning direction toward the opposite margin. Thedies are drawing dies and once any surface area of the area of the metalis engaged by their matching faces, it remains so engaged until the endof the drawing operation so that the entire areas of the die faces arein engagement with the stock at the end of the drawing operation. Theunidimensional tensioning into the range above the elastic limit iseffected by extraneous means prior to, and in dependently of, the dieclosing operation, and preferably is continued from its inception untilthe dies are in closed position. A very brief period of dwell of thedies, after die closure, is dmirable in some instances. Withunidimensional tension, the margins of the sheet extending in thetensioning direction, are left free so that the metal of the sheet cancreep and be drawn inwardly transversely of that direction duringclosure of the dies.

As a further refinement, the tensioning may be reduced ,while the diesare closing so that the tensioning by the extraneous tensioning'meansindependently of the die closure, combined with the tensioning of themetal imposed by the engagement of the dies with the metal, maintain themetal above its elastic limit during die 4; to flow so as to eliminatewrinkles, air pockets, and the like.

Again, the drawing dies employed are distinguished from wipe formingshoes, either in the form of a friction shoe or a roller, disclosed inthe patents; Though the forming shoe is shown therein as a type of dieand engages and forms the stock progressively from one end to the otherWhile the stock is held under extraneously applied stretch formingtension, this shoe type of die is one in which only substantially linecontact is maintained with the stock at all times by the die or shoe,and each portion of the stock that is engaged is immediately released sothat, at the end of the forming operation, no part of the metal, exceptpossibly for a line of contact at the very end, is in engagement withthe face of the shoe or die.

Hereinafter, whenever reference is made to the tensioning of the stockendwise, this does not necessarily 7 wise of the stock, but not bothconcurrently.

Again, hereinafter reference is made to forming the I sheet metal stockinto an article between complementary closure. This reduces thepossibility that the tension due drawing dies by forming it to the finalshape desircd, but between male and female dies of which the entireforming surfaces at the completion of the drawing operation are incontact with the stock concurrently and the major portion of the formedarticle is within the peripheral limits of the forming surfaces. Thiswording is chosen primarily to distinguish from what is known asjoggling in which a large article is formed onto a side face die and asmall ofiset of some nature is then formed in the article. Joggling isbroadly old and is disclosed in the Bath Patent No. 2,514,830, issuedJuly 11, 1950. In such joggling practices, the joggling die and the sideface die about which the stock is stressed are not the type of matchingdrawing dies herein referred to, which are of the type in whichsubstantially the entire periphery of the article is within and confinedwithin the peripheral limits of the die faces and substantially all ofthe stock, or at least the major portion thereof, is confined betweenmatching surfaces of the dies at the end of the forming operation.

, Referring ,to Figs. 1 through 9, a preferred form of machine embodyingthe principles of the present invention is illustrated. This machinecomprises a main frame 1 which, for convenience in description, isassumed to be horizontal. The frame 1 is comprised of spaced side framemembersZ and a top frame member or table 3. A compression die or drawingdie 4 is supported on the table 3 in fixed position. In the illustrativeexample, the die 4 is shown as a male drawing die.

A complementary female die 5, having a forming face matching with theforming face of the die 4, is supported on the frame for movement intocooperative relation to the die 4 so as to form the sheet stock byengaging it between the complementary matching faces of the dies 4 and5.

The dies 4 and 5 are of the general type customarily used for drawing inconventional presses in which a sheet 'of metal is disposed between thedies and, by means of suitable hold-down devices engaging theperiphery'of the sheet, the periphery is held in fixed position whilethe major portion of the sheet is drawn by closure of the diessimultaneously over their entire matching forming surfaces by movingthem from an open position while they are held parallel to their finalclosed or forming position. Because of the periphery of the sheet beingheld infixed position, themetal is stretchedsolely by the dies.

In the present instance, the stock is tensioned unidimensionallyindependently of the dies prior to, and during the drawing operation.The stock preferably is not held at the margins extending in thetensioning direction, and the die closure is such that the matching diefaces engage the stock progressively from one margin to the oppositemargin in the tensioning direction. Several advantages are obtained bythese changes. For the purpose of supporting the die so that it can beclosed relative to the die 4 progressively from one edge of the dies tothe opposite edge, a rocker arm 7 is pivotally connected at one of itsedges by a pivot 8 to upright posts 9 on the rigid frame 1. Theseupright posts are spaced laterally of the frame and the arm 7 is formedof laterally spaced plate members 19 so that ready access is afforded tothe die space both from the end and sides of the machine.

In order to rock the arm about the pivot 8, piston and cylinderassemblages 12 are provided at the edge of the arm opposite from thepivot 8. Each assemblage 12 comprises a cylinder 13 in which isreciprocable a piston 14. The piston has a rod 15 which is connected bya pivot 16 to the end of the arm 7 opposite from the pivot 8. Eachcylinder is connected at its lower end by a pivot 17 to the frame of themachine. Two such assemblages 12 are provided and they are spaced apartlaterally of the frame of the machine at the lateral outer limits of thearm 7 so as not to obstruct access to the die space from that end of themachine. By admitting pressure fluid to the rod ends of the cylinders13, the arm 7 can be rocked downwardly about its pivot 8 to close thedies progressively from one edge to the opposite edge. By admittingpressure fluid to the head ends of the cylinders 13, the arm can beraised to starting position.

In order to stretch form the material, suitable tensioning means arearranged at opposite ends of the die 4 and are operable to tension thestock unidimensionally in a range above the elastic limit, and maintainit so tensioned before and during die closure. Both of these tensioningmeans may be the same, if desired, but for economy it is preferable thatthe stretch forming means at the edge of the dies adjacent the pivot 8be a mere gripping head mounted for swinging about a horizontal fixedpivot extending parallel to the pivot 8. At the opposite edge of thedies, a gripping head employed is connected, to swing about a horizontalpivot extending parallel to the pivot 8, to a hydraulic piston andcylinder assemblage. The assemblage is arranged to yieldably urge itsassociated stock gripping head away from the head at the opposite edgeof the dies so as to yieldably tension the stock between the headsunidimensionally as distinguished from tensioning it concurrently bothendwise and crosswise.

Referring first to the stretch forming means at the end of the machineadjacent the pivot 8, this tensioning means comprises a stretch head 20on which are detachably mounted a plurality of grippers 21. The grippers21 are arranged in a row extending transversely of the machine. They areadjustably mounted on the head 20 so that, if desired, they can form agripping throat which is curvilinear transversely of the machine andconforms approximately to the cross section of the convex curvilineardie 4 when such a die is used.

For this purpose, each gripper is swivelly connected by a swivel 22 to aT-head support 23 which is slidable vertically in suitable guideways 24in the head 20. Adjusting screws 25 respective to the supports 23 areprovided for adjusting the heights of the grippers 21 independently ofeach other. The grippers are supplied with fluid pressure throughsuitable manifolding, indicated generally at 26, so that all can beopened and closed simultaneously in a well known manner.

While the specific structure of the jaws does not form a part of thepresent invention, for a better understanding of the invention one setof jaws is illustrated in Fig. 8. Each gripper 21 is open at its forwardend and is pro vided with inwardly facing cam surf-aces 27 between whichare complementary gripping jaws 28. The jaws 28 have outer cam engagingsurfaces so arranged that when the jaws are thrust forwardly toward theopen end of their grippers 21, they are moved relatively toward eachother, and when withdrawn rearwardly from the open en they separate fromeach other for releasing the stock. The jaws are connected to a commonpiston rod 29 of a piston 3t which is reciprocable in a cylinder 31 inthe gripper 21. Pressure fluid can be supplied to opposite sides of thecylinders 31 through ducts 32 and 33 respectively, and vented therefromby the ducts, so that the piston can be forced to advance or retract asdesired for closing and releasing the jaws.

It sometimes happens that, due to the slight spacing of the grippers 28from each other endwise of the row, the stock is not tensioned to ashigh a degree at the spaces between the grippers and consequently thestock tends to develop wrinkles extending in the tensioning directionfrom these spaces partway toward the opposite edge of the stock. This isoften the case with other than relatively soft metals.

In such instances, a single gripper of the same type as the grippers 21,but coextensive in length, endwise of the row of grippers, with theentire group, may be employed. This assures that the gripping force iscontinuous for the entire length of the gripped portion of the stockand, therefore, the tensioning force is uniformly applied to eachincrement along the gripped portion. If desired, the gripping throat maybe curved transversely of the tensioning direction so that the grippedmargin of the sheet is curved transversely of the tensioning directionmore nearly approximate the corresponding section of the dies prior toapplying the stock to the die.

If only a few duplicate articles are to be formed, a like effect can beobtained by employing two continuous gripping jaws between the jaws 28,the continuous jaws extending unbrokenly for the full length of thegripped margin of the sheet and thus, in effect, providing a single ppThe entire stretch head 20 is mounted for swinging movement about ahorizontal axis by a horizontal pivot 35 by which the body is connectedto suitable supports 36 on the frame of the machine. The pivot supportsthe head 20 for swinging upwardly and downwardly so that it can alignitself and the sheet more effectively with respect to the surface of thedie 4.

The complementary stretch forming means at the opposite end of the diescomprises a head 40 to which a plurality of grippers 41, similar to thegrippers 21, are connected. The head 40 may be in the form of a singlegripper as described in connection with the head 20. The head 40, inturn, is connected, for swinging about a horizontal axis, by a pivot 42to a piston rod 44 of a piston and cylinder assemblage 45 having apiston 46 and a cylinder 47. The cylinder 47 is mounted on trunnions 48for rocking about a horizontal axis. The axes of the pivot 42 and thetrunnions 43 are parallel to the axis of the pivot 8.

The trunnions 48 are mounted in suitable upright side walls of anO-frarne 49. Thus, the gripping heads 20 and 40' can be caused to gripopposite margins of the sheet. By applying pressure fluid to the rod endof the assemblage 45, the stock can be tensioned unidimensionally to thedegree desired. The capacity of the assemblage 45 is such that it canraise the stock up to and beyond its elastic limit or even up to itsultimate strength. Usually the gripping head 40 is sufliciently low sothat the stock can be drawn over the left margin of the die 4 by thehead 20. However, the stock preferably is kept above the forming face ofthe die 4 in the initial stretching operation. For this reason, it isdesirable that the head 40 be arranged to swing downwardly after initialtensioning and before die operation, so that the stock can be stretchformed approximately progressively on ;from the right edgeto the leftedge of the dies.

the die 4 and pulled downwardly around the left margin of the die 4. Forthis purpose, the O-frame 49 for the piston and cylinder assemblage 45is mounted on a rock er arm 50. The arm 50 is connected to the frame bya horizontal pivot 51 so that the arm 50, and with it the .O -frame 49,can be swung upwardly from the solid line position illustrated in Fig.2, and downwardly therefrom at least to and below thedotted lineposition shown in Fig. 2. Space is allowed between the side framemembers 2 of the frame so that the O-frame 49 can be swung even fartherdown than illustrated.

For swinging the O-frarne 49 downwardly, apiston and cylinder assemblage52 is provided. This assemblage comprises a piston 53 having a rod 54and being reciprocable in a cylinder 55. The piston rod is connected bya pivot 56 to the arm 50. By introducing pressure fluid into the rod endof the piston, the arm 50 can be swung downwardly, and by introducingpressure fluid to the opposite "end of the piston, the rod can be raised'io'the desired startingposition. The assemblage 52 can be connected tothe frame by means of a pivot 57 so as to align itself properly duringthe swinging action of the arm 50. i

For operating the machine, pressure fluid is supplied to the parts by asuitable pump P driven by an electric motor M through a line 60 to whichare connected in parallel three-way valves61, 62 and 63, respectively.

The valve 61 is connected to the assemblages 12, the.

-valve 62 to the assemblage 45, and the valve 63 10 the assemblage 52.

In thepreferred operation of the machine, the piston :and cylinderassemblages 12 are operated to raise the :arm 7' to its uppermostposition, thus holding the dies .4 and 5 separated for their fulllength. In this condition, the sheet metal stock to be formed is grippedby the grippers of the heads 20 and 40 of the stretch forming means.With the arm still in the starting position, prior Ito any operationofthe 'dies, the pressure fluid is intro- :duced to the rod end of theassemblage 45 to apply tension to the stock up to and beyond its elasticlimit.- This tension preferably is maintained in this range above theelastic limit throughout the entire forming operation,

either by the tensioning means alone or by the tensioning means plus thefrictional pull of the dies 4 and 5, as later to be explained.

- 'While the stock is maintained at a tension above the elastic limit bythe extraneous stretch forming means acting independently of dieoperation, the assemblages .12 are operated to lower the arm 7. Sincethe arm must swing about. the pivot 8, the dies 4 and 5 are first closedat the right edge adjacent the pivot 8 and engage both faces of thestock between their complementary matching faces. While the tension isbeing held, the dies are progressively closed from the edge adjacent thepivot S toward the opposite edge adjacent, the head 41;

The die 4 is preferably upwardly convex and curvilinear both inlongitudinal and transversesection, and the die 5 is correspondinglydownwardly concave. -However, as-in many drawing dies the centralportion of the forming face of the die 5 may be-omitted, unless somecritical area need be formed thereby. Ordinarily, due to the drawingeffect of the peripheral portions of the die faces and the centralportion of the male die face, -it is not necessary to have a femalematchingportion -at the central portion of the female die as thestoclcwill be stretched and formed adequately over this central portionofthe male die so as to'take the form thereof.

Due to the gradual closure 'of the-dies progressively from one edge tothe other edge in the tensioning or 'stretchforming dimension, combinedwith the maintenance'of the'stock in plastic condition due to tensioning.it into a range above itselastic limit, the metal tends to flow in thedirection of closure, which, inFig. 2, Ais t strained and unconfined byany extraneous holding means at these edges of the dies which extend inthe tensiondirection. Since the metal can creep into the dies freelytransversely of the tensioning direction and can flow in the tensioningdirection as urged'so to do by the die faces and extraneous tensioningmeans, the metal readily takes the form of the matching faces of thedie. All wrinkles and entrapped air and lubricants and the like areironed ou in the direction of closure and eliminated. The normaltendency of the sheet metal to contract transversely of the tensioningdirection assists the flow of metal inwardly of the dies transversely ofthat direction and the conformance of the metal to the matching diefaces. It is to be noted that once any increment of the metal isinitially engaged between the matching faces of the dies, it remains soconfined therebetween throughout the remainder of the die closingoperation, so that, at the end of the die closing operation, thearticle, or the major portion of the article, is within the peripherallimits of the dies. Furthermore, due to this combined stretching and dieforming, the metal article when formed retains its shape under severevariations in conditions, such as heating to high temperatures and thelike. Instead of warping and oil canning under temperature changes orwhen subjected to extraneous stresses applied normal to its faces, itholds, or returns to, its

original shape.

If desired, the head 49 can be lowered so that the entire sheet isstretch formed approximately across the convex die 4 before initiationof the closure of the female die 5. On the other hand, if desired, thehead 40 may be arranged so that the stock is not fully laid on the die 4during stretch forming before initiating the closure of the die 5. Thestretch forming and the'closure of the die 5 may progress together sothat the stock is, in effect, suspended between the two dies by thestretch forming heads 20 and 419 from theinstantaneous point of closureto the left and thus is less restrained by frictional drag againsteither die during die closure. After the part is formed in the mannerdescribed, the heads 20 and 40 are released from the stock and thereuponthe dies 4 and 5 are opened and the article removed. I V

The method and apparatus are particularly suitable for forming thepanels described and, when so formed, they are free from what is'commonly known as oil canning, a condition in which, if any pressure isapplied from the convex side of the bulge, the panel tends to snap pastdead center in the direction of the applied pressure and presents anoutwardly concave area at the face to which the force is applied. T V

In cold forming the articles in accordance with the present invention,as herein described, the stock is given its final shape by the action ofthe drawing dies, but while it is in the more plastic or flowablecondition caused by stretching: it into the range above its elasticlimit immediately before and during closure of the drawing dies.

As heretofore mentioned, it is possible to stretch the metal above theelastic limit so as to render it plastic and, within a very few secondsand before it work hardens, form it in the compression dies. This isbecause the metal, when once stretched beyond its'elastic limit, remainsfor a period of some few seconds in the plastic or fiowable condition sothat it can be more readily formed. Therefore, it can be moved from thestretchforming means and placed in a separate press die and formed whilein this condition with some advantages. While certain advantages overconventional'die drawing can 9 multidimensionally, then it cannot flowas readily into the dies transversely of the tensioning direction, andair is entrapped, wrinkles formed, and the resultant article does nothold its shape as well. The lateral tensioning of the stock in thepresent instance is that imposed by the drawing action of the ies.

Preferably, the tensioning is maintai ned above the elastic limit untilthe dies are fully closed, as mentioned. In many instances, while thedies are held closed on the article, the stock may be given anadditional restretch by suddenly increasing pressure in the stretchforming or tensioning piston and cylinder assemblage. If suchrestretching is desired along both ends of the stock, the head 24 can beoperated by a like stretch forming assemblage as that of the head 49, sothat with the dies closed, the end margins of the stock can be stretchedin opposite directions while the mid-position of the stock is heldfrictionally in fixed position by the dies. This restretch tends to setthe material around the margins.

In the form illustrated, there is a relatively shallow bulge over theentire article and the closure of the dies does not greatly increase thetension on the stock. However, in some instances, the dies themselvesduring closure do impart a considerable amount of tension. The combinedeffect may result in tensioning the stock both laterally andlongitudinally. If this die imposed tensioning is appreciable in thedirection of independently applied tension, it can be allowed for in thetensioning of the stock by the extraneous tensioning means, such as theheads 20 and 40. When this is done, the tension to raise the stock abovethe elastic limits is determined and generally is provided by the heads20 and 49. However, if the closure of the dies adds an appreciabletension, which might raise the stock up to its ultimate strength or toohigh in the range above the elastic limit, then the initial tensioningis reduced during closure of the dies so that the tension applied by theheads 20 and 40, plus the tension imparted by the dies themselves duringclosure, maintains the stock above the elastic limit but below itsultimate.

Referring next to Figs. 10 through 12, a modified form of the machine isillustrated, this machine functioning in essentially the same manner asthat heretofore described. The modified machine comprises a rigid frame7% on which is mounted a male die 71 and a complementary female die 72,these dies being of compression drawing type. The dies may be mounted oncarriers as illustrated. They are arranged so that the extraneoustensioning is in the direction of their short dimension, and they areclosed progressively in the same direction. However, they are connectedtogether at one edge by a pivot 73 so that they can swing from open toclosed position the same as the dies 4 and 5 heretofore described.Mounted at opposite edges of the dies, respectively, are the extraneousstretch forming means which can tension the stock into a range above itselastic limit independently of operation of the dies. Each stretch unitof these stretching means is the same in form and function, so only theone at the left end of the machine will be described specifically.

Each stretch unit comprises a gripping head 75 which may correspond tothe heads 20 and 40 heretofore described. Each head 75 is pivotallyconnected by a pivot 76 to a rock lever 77 which is rockable about ahorizontal axis. For rocking the lever, a piston and cylinder assemblage78, including apiston 79 and a rod 80, is provided. The rod 80 ispivotally connected by a pivot 81 to the arm 77 and pivotally connectedby a pivot 82 to the frame of the machine. Thus the stock can begripped, stretched, and swung downwardly at its gripped margins so thatit is stretch formed to conform approximately to the upper face of thedie 71.

In order to close the dies, suitable piston and cylinder assemblages 83are provided for lifting the dies to open position for permittinginsertion of the stock. The as- 10 semblages may be air operated andsubject to control to ofier a slight yieldable resistance to closure ofthe dies.

In order to close the dies, an upright support is mounted on the frameand carries a pressure roll 86. which is urged downwardly under heavypressure by a piston and cylinder assemblage 87. The roll is mounted invertical slides 38 which are slidable vertically in guide-' ways 89 inthe support 85. The support 85 itself is mounted on suitablelongitudinal guideways 89 on the frame 70 for movement lengthwise of thedies 71 and 72. For moving the support 85 lengthwise of the dies, acylinder 90 is connected to the support 35. A double end piston 91, withrods 92 which are secured at their ends to the frame of the machine, isreciprocable in the cylinder 90. Thus by introducing pressure toopposite sides of the pistons, selectively, the support 85 can bereciprocated endw-ise of the machine. Concurrently with thisreciprocation, pressure fluid can be introduced into the cylinders 87for forcing the roll 86 downwardly to apply the required pressure to thetop of the die 72.

These operations are begun with the roller at the right edge of the diesadjacent the pivot 73 and are continued with the roller yieldablypressed against the top of the die 72 as the support 85 is moved to theleft edge of the dies. This provides the desired progressive closure ofthe dies and application of forming or drawing pressure.

After the roll 86 has reached the left edge of the dies 72, the pressurein the assemblages 7% is increased if restretch is desired, and, if not,is reduced. In any event, it is finally relieved, the gripping heads 75are then released, and thereupon the roll 86 is lifted and moved to theright end of the dies and the dies are opened and the article removed.

As illustrated in Fig. 12, it sometimes happens that it is desirable toform the piece in dies which are the reverse of those indicated in Fig.11 in that a concave female die 94 is carried on the bed of the machineand the pivoted male die 95 is moved by the pressure applying means intoengagement. In suchcase, the stock is stretched above its elasticlimit,'across thecavity of the female die, and thereupon thedie 95 islowered, all in the manner hereto fore described.

Having thus described my invention, I claim:

1. The method of cold forming sheet metal stock into an article betweencomplementary matching male and female drawing dies of which the formingsurfaces are curvilinear in both longitudinal and transverse crosssection and together define a concavo-convex pattern for the article,and comprising tensioning the stock along only one dimension into arange above its elastic limit to place it in more readily deformablecondition, preparatory and prior to,a nd independently of, theinitiation of the drawing of the stock by the dies, th reafter, whilemaintaining the stock in said condition, drawing the stock bothlongitudinally and transversely to final shape by and between said dies,and maintaining each surface portion of the stock, after said surfaceportion is initially engaged by the dies, so engaged until the end ofthe drawing operation so that the surfaces of the dies in theirentirety, at the completion of the drawing operation, are in contactwith the stock concurrently, and the major portion of the resultantformed article lies within the peripheral limits of the formingsurfaces.

2. The method according to claim 1 wherein the independent tens'ioningalong said one dimension of the stock is effected by gripping oppositemargins of the stock, and the drawing by the dies is effectedprogressively along said dimension from one of said margins to theother.

3. The method according to claim 1 wherein during the independenttensioning along said one dimension of the stock and the maintenance ofthe stock in said condition and the drawing of the stock by the dies,the lateral margins of the stock extending along said dimension are leftfree to be pulled in transversely of that dimension by the d e rawin peo by the dies is progressive from the margin of one set to the oppositemargin of the same set.

5. The method according to claim gl wherein, after the stock ispartially drawn by the dies, the independent tensioning is reduced sothat the tensioning imposed by drawing by the dies combined with thereduced independent tensioning, maintain the stock within said range andbelow its ultimate strength during the remainder of the drawingoperation. 7

6. The method according to claim 1 wherein the independent tensioningalong said one dimension of the sheet stock is effected by grippingopposite margins of the stock, and after the stock is partially drawn bythe dies, the independent tensioning is reduced to a degree below saidrange, but suflicient to constrain the stock from being pulled into thedies from the outside of, the dies in a direction from' one of saidmargins toward the other during the remainder of the drawing operation.

' 7. The method of cold forming sheet metal stock into an articlebetween complementary matching male and female'drawing dies of'which theforming surfaces, are curvilinear in both longitudinal and transversecross section and together define a concavo-convex pattern for thearticle, comprising tensioning the stock along only one dimension into arange above its elastic limit'to place it more readily deformablecondition, preparatory and prior to and independently of the initiationof the draw ing of the stock by the dies, and thereafter whilemaintaining the stock in said condition and allowing it to be 7 pulledinwardly of the dies transversely of said one dimension, drawing the'stoclrboth longitudinally and transverse ly partially into final shapeby and between said dies, then discontinuing the independent tensioning,and drawing the stock to final shape in the absence of said independenttensioning, and maintaining each surface portion of the stock, aftersaid surface portion is initially engaged by the dies, so engaged untilthe end of the drawing operation, so that the surfaces of the dies intheir entirety, at the completion of the drawing operation, are incontact with the stock concurrently, and the major portion of theresultant formed article lies Within the peripheral limits of theforming surfaces.

8. The method according to claim lwherein, after the completion of thedrawing operation and while the stock is fully enclosed in'the dies, theindependent tensioning in the range above the elastic limit of the stockis temporarily increased.

9. The method of cold forming sheet metal stock hava ing two'sets ofopposite margins into an article between condition, preparatory andprior to, and independently 0f, the initiation of the drawing of thestock by the dies,

thereafter, while maintaining the stock in said condition, stretchforming the stock over the male die partially into q'fin-al shape of thearticle while allowing the 'margins of the other set, which extend alongsaid dimension, to be pulled freely inwardly transversely'of saiddimension, then, while maintaining the tensioning and allowing saidmargins of said'other set to be pulled in freely, drawing the'stock bothlongitudinally and transversely into final :shape by and between saiddies, and maintaining each surface portion of the stock, after saidsurface portion ;is 'initiallyiengaged by the dies, so engaged until theend of the drawing operation so that the surfaces of the dies in theirentirety at the cornpletionfof the drawing operation are in contact withthe stock concurrently, and

the major portion of the resultant formed article lies within theperipheral limits of the forming surfaces.

10. The method according to claim 9 wherein the drawing by the dies iseffected progressively along said dimension from one margin of said oneset to the other margin of said one set. V

11. :In a sheet metal forming apparatus, a frame, complementary male andfemale drawing dies carried thereby and having matched forming surfaceswhich are curvilinear in both longitudinal and transverse cross sectionand together define a concavo-convex pattern for the article, and whichare adapted to engage opposite faces of sheet metal stock which has twosets of opposite margins and to form it into an article the shape of thepattern so that the major portion of the article is within the outermostperipheral limits of the forming surfaces, and the surfaces in theirentirety are in contact with the stock concurrently when the dies are.fully closed, means supporting the dies on the frame for movement toopen .and closed positions so as to maintain each surface portion of thestock, after said portion is initially engaged by the dies,.so engageduntil the end of the die forming operation, power means operable toclose the dies, stretch forming means, including grippers at oppositemargins of one set connectable to said last mentioned margins so as toapply tension to the stock for tensioning the stock, in only thedimension extending from one margin of said one set to the other marginof said one set, into a range above its elastic limit while the stock isbetween the dies and the dies are open, and for maintaining the stocktensioned in said range during closure of the dies,-and said apparatusbeing free from means which can restrain the margins of the other setfrom being pulled freely in transversely of the dies by the dies as thedies are closed.

' 12. The apparatus according to claim 11 wherein means are providedwhich constrain the dies to close progressively in a direction from onemargin of said one set to the other margin of the said one set.

13. The apparatus, according to claim 12 wherein said dies are pivotallyconnected together at the margins of the dies adjacent to one of themargins of said one set for rocking from open to closed position toeffect said progressive closure.

14. The method of cold forming sheet metal stock into an article betweencomplementary matching male and female drawing dies of which the formingsurfaces are curvilinear in both longitudinal and transverse crosssection and together define a concave-convex pattern for the article ofa depth-to-area ratio too shallow to permanently deform the metal tofinal shape by closure of the dies, and comprising tensioning the sheetstock along only one dimension into a range above its elastic limit toplace it in more readily deformable condition preparatory and prior to,and independently of, the initiation of the drawing of the stock by thedies, thereafter, while maintaining the stock in said condition, drawingthe stock'both longitudinally and transversely by and between saiddiesinto a final shape wherein its depth is so :shallow relative to its areathat, in the absence of the independent stretching, it-would remain'inthe elastic range and not permanently deform into said final shape, andmaintaining each surface portion of the stock, after said surfaceportion is initially engaged by the dies, soengaged until the end of thedrawing operation, so that the surface of the dies in their entirety, atthe completion of the drawing operation, are in contact with the stockconcurrently, and the major portion of the resultantformed article lieswithin the peripheral limits of the forming surfaces.

References Cited in the file of this patent V UNITED STATES PATENTS2,619,149. .Self. Names, 1952 2,858,070 Castleton et al Sept. 2, 19582,850,071

Kraybill Sept. 2, 1958

